Substantial hysteresis in emergent temperature sensitivity of global wetland CH4 emissions

Wetland methane (CH4) emissions (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${F}_{{{CH}}_{4}}$$\end{document}FCH4) are important in global carbon budgets and climate change assessments. Currently, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${F}_{{{CH}}_{4}}$$\end{document}FCH4 projections rely on prescribed static temperature sensitivity that varies among biogeochemical models. Meta-analyses have proposed a consistent \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${F}_{{{CH}}_{4}}$$\end{document}FCH4 temperature dependence across spatial scales for use in models; however, site-level studies demonstrate that \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${F}_{{{CH}}_{4}}$$\end{document}FCH4 are often controlled by factors beyond temperature. Here, we evaluate the relationship between \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${F}_{{{CH}}_{4}}$$\end{document}FCH4 and temperature using observations from the FLUXNET-CH4 database. Measurements collected across the globe show substantial seasonal hysteresis between \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${F}_{{{CH}}_{4}}$$\end{document}FCH4 and temperature, suggesting larger \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${F}_{{{CH}}_{4}}$$\end{document}FCH4 sensitivity to temperature later in the frost-free season (about 77% of site-years). Results derived from a machine-learning model and several regression models highlight the importance of representing the large spatial and temporal variability within site-years and ecosystem types. Mechanistic advancements in biogeochemical model parameterization and detailed measurements in factors modulating CH4 production are thus needed to improve global CH4 budget assessments.

year (i.e., intra-seasonal variability), among frost-free seasons within the same ecosystem site (i.e., inter-annual variability), among sites within the same ecosystem type (i.e., inter-site variability), and among ecosystem types The results inferred from earlier and later parts of the frost-free season, and full frost-free season are colored in red, blue, and black, respectively. Start and end dates represent the beginning and ending of the frost-free season, respectively. ! and ! denote the mean seasonal CH 4 emission hysteresis and normalized area of seasonal CH 4 emission hysteresis calculated in each site-year, respectively.
Supplemental Figure 20. The distribution of seasonal CH 4 emission hysteresis inferred from the period when gross primary productivity (GPP) is above zero is consistent with the patterns found using data collected during the frost-free season. The distribution of mean seasonal CH 4 emission hysteresis (a) and normalized area of seasonal CH 4 emission hysteresis (c) to air temperature among site-years derived from the FLUXNET-CH 4 database, when GPP > 0. Positive seasonal CH 4 emission hysteresis indicates higher CH 4 emission later in the frost-free season at the same air temperature (e.g., Figure 1). Red dashed lines represent the y-axis in each data group (i.e., no hysteresis). The corresponding boxplot of site-year specific mean seasonal Supplemental Figure 21. The distribution of seasonal CH 4 emission hysteresis inferred from the period when gross primary productivity (GPP) is above 5% of annual GPP maximum is consistent with the patterns found using data collected during the frost-free season. The distribution of mean seasonal CH 4 emission hysteresis (a) and normalized area of seasonal CH 4 emission hysteresis (c) to air temperature among site-years derived from the FLUXNET-CH 4 database, when GPP > 5% of annual GPP maximum. Positive seasonal CH 4 emission hysteresis indicates higher CH 4 emission later in the frost-free season at the same air temperature (e.g., Figure 1). Red dashed lines represent the y-axis in each data group (i.e., no hysteresis). The corresponding boxplot of site-year specific mean seasonal CH 4 emission hysteresis (b) and normalized area of seasonal CH 4 emission hysteresis (d) derived from the FLUXNET-CH 4 database. The red central mark, and the bottom and top edges of the blue box indicate the median, and the 25 th and 75 th percentiles, respectively. The black whiskers extend to the most extreme data points not considered outliers denoted in red plus symbol(s).
Supplemental Figure 22. Substantial GPP values are detected when air temperatures are well below 0 ˚C.
Density scatter plots between gross primary productivity (GPP) and air temperature measured at the bog (a), fen (b), marsh (c), peat plateau (d), rice paddy (e), salt marsh (f), swamp (g), and wet tundra (h) sites, when gross primary production (GPP) is greater than 5% of annual GPP maximum.
Supplemental Figure 23. The distribution of seasonal CH 4 emission hysteresis inferred from non-zero CH 4 emissions at 0 °C is consistent with the patterns found assuming no CH 4 emissions at 0 °C. The distribution of normalized area of seasonal CH 4 emission hysteresis ( ! ) to air temperature among site-years derived from the FLUXNET-CH 4 database, when mean CH 4 emission between -0.5 to 0.5 °C is used to represent CH 4 emissions at 0 °C (a). Positive seasonal CH 4 emission hysteresis indicates higher CH 4 emissions later in the frost-free season at the same temperature (e.g., Fig. 1d to 1f). Red dashed lines represent the y-axis in each data group (i.e., no hysteresis). The corresponding boxplot of site-year specific ! derived from the FLUXNET-CH 4 database (b). The red central mark, and the bottom and top edges of the blue box indicate the median, and the 25 th and 75 th percentiles, respectively. The black whiskers extend to the most extreme data points not considered outliers denoted in red plus symbol(s).